This invention relates generally to an electrophotographic printing machine, and more particularly concerns an improved transfer apparatus for utilization therein.
As described in U.S. Pat. No. 2,297,691 issued to Carlson in 1942, the process of electrophotographic printing requires that a photoconductive member be charged to a substantially uniform potential so as to sensitize the surface thereof. Thereafter, the charged photoconductive surface is exposed to a light image of an original document. Exposure of the charged photoconductive surface to the light image selectively dissipates the charge in the irradiated areas in accordance with the light intensity projected thereon. This records an electrostatic latent image on the photoconductive surface corresponding to the original document. Development of the electrostatic latent image recorded on the photoconductive surface is achieved by bringing a developer mix into contact therewith. A typical developer mix generally employs dyed or colored thermosplastic powders, known in the art as toner particles, which are mixed with coarser carrier granules, such as ferromagnetic granules. The toner particles of the developer mix are selected to have the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. As the developer mix is brought into contact with the electrostatic latent image, the greater attractive force thereof moves the toner particles from the carrier granules to the latent image. Thereafter, the toner powder image is transferred from the electrostatic latent image to a sheet of support material, such as paper, or a thermoplastic sheet, amongst others. The toner powder image adhering to the sheet of support material is permanently affixed thereto by the application of heat.
Hereinbefore, the toner powder image has been transferred to the sheet of support by an electrical field created by a corona generator similar to that disclosed in U.S. Pat. No. 2,836,724 issued to Vyverberg in 1958. The corona generator induces transfer of the toner particles to the sheet of support material by spraying a corona discharge having a polarity opposite to that carried by the toner particles on the photoconductive surface onto the backside of the sheet of support material. This causes the toner particles to be electrostatically transferred to the sheet of support material.
Other techniques utilize electrically biased transfer rolls. The transfer roll generates a high voltage discharge in the proximity of the surface of the paper, or it may be applied by means of a conductive cylinder in contact with paper as disclosed in U.S. Pat. No. 2,807,233 issued to Fitch in 1957. As taught therein, a sheet of support material is interposed between the conductive roller and a surface having the toner powder image thereof. A charge of opposite polarity from that of the toner particles is deposited on the backside of the sheet of support material which attracts the toner powder image thereto.
Finally, U.S. Pat. No. 3,838,918 issued to Fisher et al. in 1974 describes a corona generator with pre-conditions the toner particles prior to their being transferred from the photoconductive surface to a sheet of support material by a transfer roll. The sheet of support material is secured to an electrically biased transfer roll. After being pre-conditioned by an alternating charge potential applied to the photoconductive surface by the corona generating device, the toner particles are transferred to the sheet of support material secured releasably on the electrically biased transfer roll.
Premature transfer of toner powder images from the photoconductive surface to the sheet of support material is minimized by the use of a control baffle. U.S. Pat. No. 3,850,519 issued to Weikel, Jr. describes a control baffle for directing the sheet of support material into contact with the photoconductive surface. The baffle is positioned adjacent to the photoconductive surface in close proximity therewith so that the sheet of support material is guided into tangential moving contact with the moving photoconductive surface prior to the sheet entering the corona stream.
U.S. Pat. No. 3,620,617 describes a corona generator having a mesh sheet interposed between the corona generating device and the sheet of support material adjacent the photoconductive member.
Another patent of interest is U.S. Pat. No. 3,785,816. This patent describes an apparatus for eliminating corona arcing between the electrodes in an electrophoretic imaging system. Baffles are positioned at the exit and entrance of the nip between adjacent electrodes at the point of image transfer. This eliminates premature arcing breakdown.
Many factors influence the quality of the transferred powder image, the most significant factors being those which effect the uniformity with which the toner powder image is transferred from the photoconductive surface to the sheet of support material. Hereinbefore, the process of transferring successive layers of toner to a common sheet of support material, as exemplified by multi-color electrophotographic printing, has posed numerous problems. In particular, when a bias transfer roll is utilized to transfer successive toner powder images, in superimposed registration, to a sheet of support material, hollow characters frequently occur. Hollow characters may be defined as a toner area wherein substantially only the periphery thereof is transferred while the central portion remains devoid of toner particles. The problem of hollow characters is most pronounced for line copy reproductions. However, hollow characters frequently occur in solid area copy as well. It has been found that without the use of a corona generating device in conjunction with a bias transfer roll, approximately sixty to seventy percent of the image is devoid of toner particles, i.e. the hollow character is of a significant magnitude. Contrawise, with the introduction of the corona generating device, the devoid area was reduced to approximately twenty to thirty percent. However, this level has proven to be unsatisfactory when high quality reproductions are desired.
Accordingly, it is the primary object of the present invention to improve the apparatus for transferring a toner powder image from a photoconductive surface to a sheet of support material.